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J. Min. Metall. Sect. B-Metall. 50 (1) B (2014) 5 - 13 Journal of Mining and Metallurgy, Section B: Metallurgy

ANTIMONY PRODUCTION BY CARBOTHERMIC REDUCTION OF STIBNITE IN THE PRESENCE OF LIME

R. Padilla a,* , L.C. Chambi b, M.C. Ruiz a

a University of Concepcion, Department of Metallurgical Engineering, Concepción, Chile b Universidad Mayor de San Andres, Department of Metallurgical and Materials Engineering, La Paz, Bolivia

(Received 04 June 2013; accepted 12 February 2014) Abstract

Experimental work on the carbothermic reduction of Sb 2S3 in the presence of lime was carried out in the temperature range of 973 to 1123 K to produce in an environmentally friendly manner. The results demonstrated the technical feasibility of producing antimony by this method without producing SO 2 gas. Complete conversion of Sb 2S3 was obtained at 1023 K in about 1000 seconds and at 1123 K in less than 250 seconds using stibnite--lime mixtures with molar ratios

Sb 2S3:CaO:C = 1:3:3. It was found that the reduction proceeds through the formation of an intermediate SbO 2, which is subsequently reduced by CO(g) to yield antimony metal and CaS. The kinetics of the Sb 2S3 reduction was analyzed by using the equation ln(1-X) = -kt. The activation energy was 233 kJ mol -1 in the temperature range of 973 to 1123 K. This value would correspond to an antimony catalyzed carbon oxidation by CO 2.

Key words: Antimony sulfide; Stibnite; Carbothermic reduction; Direct reduction.

1. Introduction capture. These studies included the direct reduction of , , , , and sulfides Stibnite (Sb 2S3) is an abundant antimony bearing [3-14 ]. and thus it is the main source for the The main experimental technique used by most production of pure and/or metallic researchers to determine the rate of reduction was antimony. The conventional technology for the thermogravimetric analysis, which was usually production of metallic antimony from stibnite complemented with several analytical techniques. concentrates is primarily by oxidizing roasting to Concerning the use of gaseous reductants, Sohn and convert the stibnite into volatile antimony oxide Won [ 3] studied the reduction of cuprous (Sb 2O3), which is subsequently reduced to antimony sulfide in the presence of lime. In this case, the global metal with carbon [ 1]. Although this conventional reaction involved the hydrogen reduction of the technology to produce Sb 2O3 is well established, the sulfide to metal with the formation of H 2S and a control of the temperature and the air draft in the subsequent reaction of the H 2S with CaO. These roasting/smelting stages to produce Sb 2O3 without researchers modeled the successive gas-solid producing the non-volatile SbO 2 is difficult [2 ]. In reactions in a porous pellet incorporating the intrinsic general, in the roasting/smelting of stibnite, the kinetics of the individual reactions, and compared the production of noxious antimony vapors and results with experimental values obtained using dioxide (SO 2) is unavoidable; therefore, this process chalcocite and lime. Mankhand and Prasad [ 4] studied has serious problems of atmospheric contamination. the hydrogen reduction of molybdenite in the The direct reduction of metal sulfides in the presence of lime and found that lime enhanced presence of lime is a viable alternative method to tremendously the rate of reduction and reduced the produce metal values without emitting the H2S concentration in the off gas to negligible values. contaminant . Several studies have been Mohan et al. [ 5] reported the results of the lime reported in the past which were concerned with the enhanced reduction of cuprous use of various reducing agents, namely hydrogen, sulfide. The reduction results were analyzed by using carbon monoxide and carbon for the reduction of diffusion through product layer control model, and metal sulfide, using mainly CaO and CaCO 3 for sulfur determined an activation energy of 169.6 kJ/mol for

* Corresponding author: [email protected]

DOI:10.2298/JMMB130604003P 6 R. Padilla et al. / JMM 50 (1) B (2014) 5 - 13 the range of 1123 to 1273 K. also reduces the sulfur dioxide concentration in the off On the other hand, the use of solid carbon as a gases to negligible values. Therefore, this alternative reducing agent for metal sulfides has received process seems appropriate for the treatment of stibnite considerable attention. Rao and El-Rahaiby [ 6] concentrates especially for small to medium size studied the reduction smelting of lead sulfide with C plants frequently found in developing countries. and CaO in the temperature range 1068 to 1255 K. Therefore, this paper is concerned with the The use of several catalysts on the rate of reduction carbothermic reduction of stibnite in the presence of was also studied. A 10-fold increase in the rate was lime. observed for the catalyzed reduction in some cases Little research has been done on this specific using K, Na and Rb carbonates. Bronson and Sohn [ 7] topic. The most relevant work was carried out by studied the feasibility of reducing nickel sulfide with Igiehon et al. [ 15 ], who studied the reduction of carbon and lime and found a rapid kinetics for the antimony sulfide with coal and graphite and proposed production of metallic nickel in the temperature range the following sequence of reactions for the reduction of 1273 to 1373 K. The carbothermic reduction of of stibnite with coal and lime: zinc sulfide has been studied using lime [ 8, 9] or Sb 2S3(s) + 3CaO(s) → Sb 2O3(s) + 3CaS(s) (1) carbonate [ 10 ], as the sulfur scavenger. The Sb 2O3(s) + 3C(gr) →2Sb + 3CO(g) (2) results showed that the reduction reaction was rapid in In reaction (1), called the exchange reaction, the the presence of catalyzers such as Na SO or NaF [ 8] 2- 2 4 S of the stibnite are basically exchanged with and carbonate [ 9]. The use of calcium 2- O from the to form antimony trioxide. carbonate as sulfur scavenger instead of lime was also The following step, reaction (2) is the reduction of the effective; however, it prevented an accurate formed antimony trioxide with carbon. The overall measurement of the rate of reduction by reaction proposed by Igiehon et al. [ 15 ] for the Sb S thermogravimetric analysis due to the decomposition 2 3 reduction with carbon and CaO was as follows: of the carbonate in the reaction system. Wu et al. [ 10 ] studied also the carbothermic reduction of zinc sulfide Sb 2S3(s) + 3CaO(s) + 3C(gr) → using calcium carbonate and based only on relative 2Sb(s,l) +3CaS(s)+ 3CO(g) (3) intensity of diffraction peaks of XRD analysis of However, the formation of Sb 2O3 as an reduction products, proposed a complex reaction intermediate reaction product in the carbothermic mechanism involving four stages and several reduction of stibnite has not been corroborated sequential and parallel reactions. In 1997, the experimentally; consequently, the mechanism of the reduction of molybdenite with carbon in the presence carbothermic reduction of stibnite still needs some of lime was studied by Padilla et al. [ 11 ], who clarification. Furthermore, the production of solely reported that the reduction proceeded through the CO(g) as the gaseous product under a variety of formation of intermediate MoO 2 and calcium experimental conditions, as indicated by reaction (3), molybdate and using a first order kinetic model, they also needs corroboration. Therefore, a more general determined an activation energy of 218.8 kJ/mol for overall reaction that could represent better the direct the molybdenite reduction with carbon in the reduction of stibnite using carbon as the reductant in temperature range of 1173 to 1473 K. Concerning the presence of lime can be written as: copper sulfides, Jha et al. [ 12 ] and Padilla and Ruiz Sb 2S3(s,l) + 3CaO(s) + (3-z)C(s) → [13 ] studied the reduction of Cu 2S with carbon and lime. The later investigators determined the 2Sb(s,l) +3CaS(s)+(3-2z) CO(g) + zCO 2(g) (4) composition of the gas phase evolved during the with 0 £ z £ 1.5. carbothermic reduction of Cu 2S, which indicated that This general reaction (4) allows for the possible the controlling step was the Boudouard reaction. An emission of mixtures of CO(g) and CO 2(g) during the activation energy of 363 kJ/mol was calculated for the carbothermic reduction of stibnite in the presence of reduction of Cu 2S with carbon and lime in the range CaO, and it reduces to reaction (3) as a limiting case of 1173 to 1323 K. Recently, Sundarmurti at al. [ 14 ] when z is equal to zero and to reaction (5) when z is reported a study on a thermal analysis of lime added equal to 1.5. carbon reduction of chalcopyrite. These investigators Sb 2S3(s,l) + 3CaO(s) + 1.5C(s) → claimed that chalcopyrite decomposes to copper and 2Sb(l) + 3CaS(s) + 1.5CO 2(g) (5) sulfides and each are reduced in distinct stages. The reabsorption of evolved sulfurous gases on lime The actual relative amounts of the two carbon was indicated as the reason for the observed weight evolved at a given experimental condition will gain of the sample. depend on the thermodynamics and the relative Generally, the main advantages of the reduction of kinetics of the intermediate reactions that occur in the metal sulfides in the presence of lime are that lime not reduction process. only enhances tremendously the rate of reduction but Considering the above, in the present work, a R. Padilla et al. / JMM 50 (1) B (2014) 5 - 13 7 systematic study of the carbothermic reduction of the gaseous products of the reaction. The sample Sb 2S3 in the presence of lime was carried out to temperature was constantly measured by a determine the mechanism and kinetics of this process, chromel/alumel thermocouple placed just below the and to assess the potential of this method as a more crucible. To check the development of local efficient and cleaner alternative for the production of temperature changes after insertion of the crucible in metallic antimony than the current industrial practice. the reactor, few experiments were conducted with a thermocouple placed inside the crucible in direct 2. Experimental contact with the reacting sample and with the 2.1 Materials thermocouple in empty crucible. The results indicated that the difference in the temperature profile for both The reduction experiments were conducted using cases was negligible for the small 500-mg sample Sb 2S3 obtained from Aldrich Chemicals Co. which used, and the sample heated rapidly to the set was a fine powder of 95.5% Sb 2S3. The reducing agent temperature in about 70 to 100 s, depending on the set used was activated carbon (charcoal activated GR, temperature for the experiment. Thus, the weight loss Merck). This carbon was 94% -75 micron and the of the sample was recorded and analyzed considering determined fixed carbon was 90.7%. The calcium the indicated heating time. oxide for the experiments was produced by thermal The fraction of weight loss at any time was decomposition of calcium carbonate powder (99.9 % calculated from the instantaneous weight of the CaCO 3, Merck). Various samples were prepared by sample. The intermediate and final compounds mixing these materials thoroughly in a porcelain formed during the reaction were determined by X-ray mortar in predetermined molar ratios of Sb 2S3:CaO:C. diffraction (XRD) analysis of partially and completely reacted samples using a Rigaku model Geiger Flex, 2.2 Reduction experiments copper anode spectrometer (Rigaku Corporation, The Woodlands, TX, USA). For this analysis, when the The reduction of Sb 2S3 ( 823K) was sample reacted for the predetermined time, the studied in a custom built thermogravimetric apparatus crucible with the sample was quickly lifted to the (TGA) where the instantaneous weight of the sample upper end of the reaction tube, where it was quenched and the temperature were recorded as a function of with a cold flow of , and then, the sample was time. In order to obtain quantitative information on the extent of reaction from the weight loss data of the reacting samples, it was necessary to exclude gaseous from the system, thus the experiments were carried out under an inert atmosphere using high grade nitrogen. A schematic drawing of the experimental apparatus is shown in Figure 1. The thermogravimetric apparatus consisted principally of a high temperature vertical tube furnace with a temperature controller, an electronic balance connected to a personal computer for recording continuously the weight of the sample, and a nitrogen delivery system to provide the inert atmosphere in the reaction tube. A quartz tube 45 mm inner diameter was used as a reactor to hold a 5 ml (20 mm high and 23 mm upper diameter) crucible containing the sample. A typical experiment started by heating the tube reactor to the selected temperature under a slow flow of nitrogen. Once the set temperature was reached and stabilized, about 500 mg of a homogeneous mixture Figure 1. Schematic diagram of the TGA system. 1. Tubular of Sb 2S3-CaO-C contained in a loosely covered furnace; 2. Furnace temperature controller; 3. crucible was introduced in the quartz tube very Chromel/alumel thermocouple; 4. Sartorious rapidly (in 10 to 20 s approximately), and it was precision balance; 5. Gold chain; 6. Quartz reaction tube; 7. Ceramic crucible; 8. bead suspended from the balance by a gold chain. The use bed; 9. Chromel/Alumel thermocouple; 10. 12- of a cover that did not fit tightly (there was a side slit channel scanning thermocouple thermometer; between the cover and the crucible to allow flow of 11. Nitrogen gas cylinder; 12. Flow meter; 13. gas) was to eliminate back diffusion and dilution of Gas inlet tube; 14. Rubber Stopcock; 15. Personal computer. 8 R. Padilla et al. / JMM 50 (1) B (2014) 5 - 13 rapidly removed to a desiccator for final cooling. Both reacted samples were analyzed by X-ray Most of the experiments were conducted at constant diffraction spectroscopy to determine the compounds temperature under a constant flow of nitrogen until formed during the experiments. The XRD spectrum of little weight change of the sample was observed. A the Sb 2S3-CaO sample reacted for 2400 s at 1073 K is few experiments were interrupted at short times in shown in Figure 3, where it can be observed strong order to determine the intermediate compound formed diffraction lines for SbO 2 (cervantite), CaS, and by the reaction. metallic Sb. This result suggests that at 1073 K in the absence 3. Results and Discussion of carbon stibnite is converted into cervantite and 3.1 Reaction products liquid antimony (Sb melting point 903.5 K) and CaO is transformed into CaS by the following reaction: Preliminary experiments were run to check the 2Sb 2S3(l) +6CaO(s) → adequacy of the custom TGA using carbon (coconut Sb(l) +3SbO (s)+6CaS(s) (6) charcoal) of known composition under various 2 gaseous atmospheres. The results of those According to this reaction (6), Sb 2S3-CaO experiments served also to define a flow rate of 0.6 L mixtures should not lose weight as the reaction min -1 nitrogen into the reactor as sufficient to maintain proceeds. Therefore, the constant weight loss of the the inert atmosphere. Thus using this nitrogen flow sample shown in Figure 2 must be due to the partial rate, several experiments were conducted in the volatilization of either initial Sb 2S3 or the product Sb temperature range of 973 K to 1123 K with since the SbO 2 is non-volatile at 1073K. Padilla et al. [16 ] in their study of volatilization of stibnite in homogeneous mixtures of Sb 2S3-CaO and Sb 2S3-CaO- C. The main purpose of these experiments was to nitrogen atmospheres found a constant volatilization determine the intermediate and final products that rate of Sb 2S3 at 1073 K, which is consistent with the could be formed when heating mixtures of stibnite, carbon and lime with various compositions. The results of the experiments obtained at 1073 K using mixtures with and without carbon, i.e., molar ratio of Sb 2S3:CaO = 1:3, and Sb 2S3:CaO:C: = 1:3:3, are shown in Figure 2. In this Figure, the weight loss fraction of the sample given by (W o-W)/W o is shown as a function of time, where W o is the initial weight and W is the weight at time t. In addition to the continuous line to represent the weight loss fraction of the sample, a symbol for intermittent data points are used mainly for clarity, especially when more than one curve corresponding to different conditions are shown in the figure. As seen in Figure 2, the sample mixture with molar ratio of Sb 2S3:CaO = 1:3 loses weight at a very Figure 2. Comparison of weight loss fraction of samples slow constant rate while the weight loss of the mixture with molar ratios of Sb 2S3:CaO = 1:3 and Sb 2S3:CaO:C= 1:3:3 is very fast at this temperature. Sb 2S3:CaO:C = 1:3:3 reacted at 1073 K.

Figure 3. Spectrum of a reacted mixture of stibnite with CaO at 1073 K and 2400 second R. Padilla et al. / JMM 50 (1) B (2014) 5 - 13 9 results shown in Figure 2. Therefore, it is believed 3.2 Reaction mechanism that the weight loss observed during the reaction of mixtures of Sb 2S3-CaO in nitrogen atmosphere is Considering the XRD results discussed above, the mainly due to the volatilization of Sb 2S3. following reaction scheme can be proposed for the On the other hand, Figure 4 shows an XRD reduction of stibnite with C in the presence of CaO for spectrum of a sample with molar ratios Sb 2S3:CaO:C the temperature range of 973 to 1123 K: = 1:3:3 which was reacted for 1250 s at the same 2Sb 2S3 (l) + 6CaO(s) → temperature of 1073 K. It can be observed in this Sb(l) + 3SbO (s) + 6CaS(s) (6) spectrum strong diffraction lines solely for Sb and 2 SbO 2(s) +C(s) → Sb(l) + CO 2(g) (7) CaS, indicating that at this temperature the Sb 2S3 reduction with carbon in the presence of lime yields Both reactions are thermodynamically favorable. only Sb and CaS as condensed products. An XRD However, despite the large value of the equilibrium spectrum of a partially reacted sample with the same constant for reaction (7) (see Table 1), it is unlikely to be the main reduction reaction of SbO in the system. molar ratios of Sb 2S3:CaO:C = 1:3:3 reacted at 948 K 2 for 2400 s is presented in Figure 5. In this spectrum, The reason is that reaction (7) is a solid-solid reaction which requires direct contact between the antimony strong diffraction peaks for SbO 2 were also found in oxide and carbon particles. Therefore, once a small addition to Sb, and CaS peaks. The presence of SbO 2 in this partially reacted sample demonstrates that the amount of Sb product is formed in the interface SbO 2- carbothermic reduction of stibnite in the presence of C, reaction (7) could only proceed further by the CaO proceeds through the intermediate compound diffusion of the reactants through this product layer which generally is a slow process. SbO 2. The formation of SbO 2 was also confirmed in samples that reacted for short times and large excess On the other hand, once some initial CO 2 is of CaO at 1023 K. produced in the system by reaction (7), the carbon

Figure 4. Spectrum of a reacted mixture of Sb 2S3:CaO:C = 1:3:1.5 at 1073 K and 1250 seconds

Figure 5. Spectrum of a reacted mixture of Sb 2S3:CaO:C = 1:3:3 at 948 K and 2400 seconds 10 R. Padilla et al. / JMM 50 (1) B (2014) 5 - 13 oxidation reaction (8) (Boudouard reaction) must take Table 1. Equilibrium constants for the reactions involved place: in the reduction of stibnite with carbon in the presence of lime [ 19 , 20 , 21 ] C(s) + CO 2(g) ↔ 2CO(g) (8) Thus, further reduction of the antimony oxide with Equilibrium constant, K Chemical Reaction CO(g) can easily occur through the gas phase T = 1000 K according to the following reaction: Sb 2S3 + 3CaO + 1.5C → 11 SbO 2 (s) + 2CO(g) → Sb(l) + 2CO 2(g) (9) 4.19x10 2Sb + 3CaS + 1.5CO 2(g) (5) Reaction (9) is a gas-solid reaction and since 2Sb 2S3 + 6CaO → diffusion in a gas phase is a rapid process (as 1.19x10 3 compared to diffusion in a condensed state), reaction Sb + 3SbO 2 + 6CaS (6) 6 (9) should be the dominant reduction reaction of SbO 2 SbO 2 + C →Sb + CO 2(g) (7) 4.87x10 in the system, as indicated by the rapid kinetics C + CO 2(g) ↔2CO(g) (8) 1.76 observed in the experimental work. Therefore, the reduction of Sb S with C and CaO would occur by a SbO 2 + 2CO(g) → 6 2 3 2.76x10 first step of oxidation to Sb(l) and SbO 2(s), according Sb + 2CO 2(g) (9) to reaction (6), followed by the reduction of SbO 2 with CO(g) as given by reaction (9). This SbO 2 reduction reaction would be coupled with the For the range of temperatures studied here, the Boudouard reaction (8) for the regeneration of CO weight losses by evaporation of the condensed species. It is a well-documented fact in the literature reagents or products are negligible. Thus, the CO that the reduction of sulphides or oxides with carbon and/or CO 2 evolution during the reduction can be occurs primarily through intermediate gaseous CO resolved from the total weight loss of the sample for complete conversion. This can be accomplished by and CO 2 species, involving the Boudouard reaction [11 , 13 , 17 , 18]. comparing the experimental weight loss data to the The equilibrium constants at 1000 K for the theoretical values calculated from the stoichiometry reactions considered in this study were calculated by of the overall reaction (5) for CO 2 evolution, and using the HSC Chemistry data base [ 19 ] for all the reaction (4) for the evolution of a mixture of CO and chemical compounds except for the Sb S . The free CO 2 gases at a given temperature considering the 2 3 Boudouard reaction equilibrium. Such a comparison energy values for Sb 2S3 given by HSC differ substantially from the values given by Barin [ 20 ] and is presented in Figure 6, which shows the fractional Pankratz et al. [ 21 ] and since the latter references give weight loss data for two experiments carried out with comparable values, the free energy of formation for molar ratios Sb 2S3:CaO:C = 1:3:3 one at 998 K and the other at 1023 K. The theoretical weight loss of the Sb 2S3 given by Barin [20] was used for calculating the equilibrium constants for reactions (5) and (6). The sample considering the formation of only CO 2 results are shown in Table 1, where it can be observed according to reaction (5) is shown as a horizontal line, that all the reactions have large equilibrium constants and the formation of a mixture of CO and CO 2 at 1000 K with the exception of the Boudouard according to the reaction (4) at 998 K and 1023 K reaction (8), which is a reversible reaction. Thus with z = 0.68, and z = 0.54, respectively, are also thermodynamics predictions are in general agreement shown in the figure. The values of z for both with the experimental results, particularly concerning temperatures were determined considering the reactions (5) and (6) of this study. equilibrium constant for the Boudouard reaction and a Therefore, according to the proposed mechanism total pressure of 1 atmosphere (P CO + P CO2 = 1) which was the case for our experimental conditions with the for the reduction of Sb S with C and CaO, the actual 2 3 sample reacting in a slackly covered crucible as composition of the evolved gas will depend on the described above. relative rates of reactions (8) and (9) at a given As can be seen in this Figure 6, at both temperature. If reaction (8) is significantly slower temperatures, the experimental weight loss than reaction (9), the gaseous product of the overall approached asymptotically to the theoretical weight reaction will be mostly CO 2, and the overall reaction loss value for the CO 2 formation only. Therefore, the will be reaction (5). On the other hand, if the SbO 2 carbothermic reduction of stibnite in the presence of reduction reaction (9) is the slowest reaction, the lime can be represented accurately by the overall composition of the gas produced will be close to the reaction (5). These results also indicate that the equilibrium composition of the Boudouard reaction Boudouard reaction is the slowest step in the (8) and a mixture of the two carbon oxides will be reduction of stibnite with carbon in the presence of evolved with a predominance of the CO at higher CaO. The Boudouard reaction has been previously temperatures, and the overall reaction will be reaction reported as the controlling reaction in the (4). carbothermic reduction of various metals oxides and R. Padilla et al. / JMM 50 (1) B (2014) 5 - 13 11 sulfides [ 11 , 13 , 17 , 18 ]; therefore, it is not Figure 7 shows that a change in temperature from unexpected this is also the case in the stibnite 973 to 1123 K produces a large increase in the rate of reduction with carbon in the presence of lime. reduction. At 1048 K and above, the reduction occurs rapidly, reaching complete reduction in less than 500 seconds. The same great effect of temperature was found when using the stoichiometric amount of carbon i.e. molar ratios of Sb 2S3:CaO:C =1:3:1.5, except that the reduction rate at each temperature was somewhat slower in this case, as shown in Figure 8.

Figure 6. Experimental weight loss fraction compared to the theoretical weight loss fraction according to the reactions (4) and (5).

3.3 Effect of temperature

At temperatures lower than 973 K the rate of Figure 8. Effect of temperature on the reduction rate for the reaction was very slow to be of practical interest. On stoichiometric mixture Sb 2S3:CaO:C = 1:3:1.5. the other hand, at temperatures higher than 1123 K the vaporization of the antimony product became 3.4 Effect of the amount of CaO noticeable and the reacting samples continued to lose weight at a slow rate with time. The results of To study the effect of the amount of CaO on the reduction experiments for samples with molar ratios overall rate of reaction, experiments were performed Sb 2S3:CaO:C = 1:3:3 (100 % excess of carbon) are at 998 K with different molar ratios of Sb 2S3:CaO shown in Figure 7, where the data are presented as the while maintaining the amount of carbon in the conversion of stibnite, X, versus reaction time. Since mixture at 3 moles of C per mole of Sb 2S3. It should the reduction of Sb 2S3 proceeds with the evolution of be pointed out that the amount of CaO in the reacting CO 2 as discussed above, the conversion was sample should affect directly the rate of formation of calculated as X = ∆W/W th , where W th is the theoretical SbO 2, i.e. reaction (6) and indirectly the rate of weight loss for the complete conversion of stibnite reduction of this oxide. The experimental results are with the formation of CO 2 according to the overall presented in Figure 9, where it can be observed that an reaction (5). increment in the amount of CaO above the stoichiometric ratio increased somewhat the overall

Figure 7. Effect of temperature on the carbothermic reduction rate of Sb 2S3 in the presence of lime. Figure 9. Effect of the amount of CaO on the rate of Sample used Sb 2S3:CaO:C = 1:3:3. reduction of Sb 2S3 with carbon. 12 R. Padilla et al. / JMM 50 (1) B (2014) 5 - 13

rate of stibnite reduction. It can also be observed that experiments with molar ratios of Sb 2S3:CaO:C = 1:3:3 higher than 4 moles of CaO per mole of Sb 2S3 is not are depicted in Figure 11. As seen in this figure, the needed since the effect is negligible. experimental data fit the kinetic model well up to a fractional conversion of approximately 0.95. The 3.5 Effect of the amount of carbon deviation observed for higher conversions was attributed to slow diffusional processes arising at the Figure 10 shows an important effect of varying the end of the reaction. amount of carbon on the rate of reduction of stibnite at 998 K using a stoichiometric amount of CaO in the mixture. A similar effect of the amount carbon on the rate of reduction was also seen at other temperatures. An increment in the amount of carbon in the sample increases the available surface area for the Boudouard reaction to take place. Therefore, if the Boudouard reaction were the controlling step, the carbon increment definitely should increase the rate of the overall reaction. Thus the observed effect of the amount of carbon on the rate of reaction further supports the assumption that the Boudouard reaction is the controlling step of the overall reaction of stibnite reduction.

Figure 11. Kinetics of the stibnite reduction with carbon in the presence of lime for samples with molar ratios Sb 2S3:CaO:C = 1:3:3.

The apparent rate constants obtained from the slopes of the straight lines in Figure 10, and similar figure for the ratio 1:3:1.5, were used to draw an Arrhenius plot, which is shown in Figure 12. In this figure, it can be observed that for the two mixtures the rate constant dependence on the temperature fits a straight line well. In addition, the slopes for both set of data are nearly identical; indicating that the controlling step of the overall reaction is the same regardless of the amount of carbon in the mixture. The Figure 10. Effect of the amount of carbon on the reduction calculated activation energy was 233 kJ/mol, for the rate of stibnite in the presence of lime. temperature range of 973-1023 K. Activation energy values reported for the uncatalyzed oxidation of 3.6 Reduction kinetics carbon by CO 2 vary from 251 to 359.8 kJ/mol [ 22-24 ], which are higher than the value determined in this The rate of the overall stibnite reduction can be described by a simple model assuming a chemical control. The following first order rate expression with respect to the fractional conversion has been found to fit the experimental data well. ln (1-X) = - kt (10) In this equation, X is the fraction of stibnite reacted at time t and k is the apparent rate constant. This kinetic expression has also been used in the carbothermic reduction of other metal sulfides and oxides [ 10 , 11 , 17 ], where the overall reduction process was controlled by the oxidation of carbon by CO 2 (Boudouard reaction). According to the proposed model, the rate constants can be determined from the experimental data shown in Figure 7 and Figure 8 by Figure 12. Arrhenius plot for the carbothermic reduction of plotting ln(1-X) versus t. Thus the data obtained for stibnite in the presence of lime. R. Padilla et al. / JMM 50 (1) B (2014) 5 - 13 13 study. On the other hand, the reported values for the noticeable. catalyzed carbon oxidation in the reduction of The kinetics of the Sb 2S3 reduction can be molybdenum sulfide [ 11 ] and lead sulfide [ 6] were analyzed by the equation ln(1-X) = -kt. The activation 218.8 and 236.8 kJ mol -1 , respectively, which are energy was determined to be equal to 233 kJ mol -1 for close to the value of 233 kJ mol -1 obtained in the the temperature range of 973 to 1123 K. present research. Therefore, this suggests that the activation energy value determined in this work References would correspond to the oxidation of carbon catalyzed by antimony metal. [1] J.G. Yang, C.B. Tang, Y.M. Chen, M.T. Tang, Metall. In overall, the results of this study have Mater. Trans. B, 42B (2011) 30-36. demonstrated the feasibility of producing antimony [2] F. Habashi, Handbook of Extractive Metallurgy Vol. II, metal from stibnite by direct reduction of the sulfide Wiley-VCH, Weinheim, New York, 1997. with carbon in the presence of lime. A major [3] H.Y. Sohn, S. Won, Metal. Trans. B, 16B (1985) 831- advantage of this method is that the stibnite 839. concentrate can be reduced to antimony at moderate [4] T. R. Mankhand, P.M. Prasad, Metall. Trans. B, 13B temperatures of 973 to 1123 K, which are (1982) 275-82. significantly lower than the temperature range of the [5] M. K. Mohan, T. R. Mankhand, P.M. Prasad, Metall. Trans. B, 18B (1987) 719-25. conventional technology practiced by industry (1423 [6] Y. K. Rao, S. K. EL-Rahaiby, Metall. Trans. B, 16B to 1623 K) [ 1, 2 ]. Therefore, the carbothermic (1985) 465-75. reduction of stibnite decreases the energy [7] M. C. Bronson, H. Y. Sohn, Metall. Trans. B, 14B requirements and lowers the losses of antimony by (1983) 605-15. volatilization. Furthermore, it will require also less [8] H. Abramowitz, Y. K. Rao, Trans. Inst. Min. Metall. gas handling and cleaning facilities than the Sect. C, 87 (1978) 180-188. conventional process due to the low volume of gas [9] Y. C. Peng, C.I. Lin, H. K. Chen, J. Mater. Sci. 42 produced, which will contain little or none SO 2. (2007) 7558-7565. Consequently, this method will also minimize the [10] C. M. Wu, C. I. Lin, H. K. Chen, Metall. Mater. Trans. ambient contamination with SO 2 due to the strong B, 37B (2006) 339-347. desulfurizing power of lime. Finally and most [11] R. Padilla, M. C. Ruiz, H. Y. Sohn, Metall. Mater, importantly, since this method comprises fewer Trans. B, 28B (1997) 265-274. chemical steps, it is suitable for the small scale [12] Jha, P. Grieveson, J.H. Jeffes, Scand. J. Metallurgy, 18 production of antimony. The post reduction treatment (1989) 31-45. to recover the antimony from the waste CaS and [13] R. Padilla, M. C. Ruiz, Can. Metall. Quart., 40 (2) unreacted compounds present in the solids was not (2001) 169-178. studied in this research. However, unit operations [14] N.S. Sundarmurti, B. R, Rehani, B.J. Rao, T. Indian I. Metals, 60 (2007) 417-430 such as grinding and gravimetric methods could [15] U. O. Igiehon, B. S. Terry, P. Grieveson, Trans. Inst. accomplish the metal separation from the other Min. Metall. Sect. C, 101 (1992) 144-154. components. [1 6] R. Padilla, G. Ramirez, M. C. Ruiz, Metall. Mater. Trans. B, 41B (2010) 1284-92. 4. Conclusions [17] R. Padilla, H. Y. Sohn, Metall. Trans. B, 10B (1979) 109-115. From the results of this study, the following can be [18] Y. K. Rao, Metall. Trans., 2 (1971) 1439-47. concluded. [19] A. Roine, HSC Chemistry 5.1, Outokumpu Research The production of antimony by directly reducing Oy, Pori, Finland. the stibnite with carbon in the presence of lime is [20] I. Barin, Thermochemical Data of Pure Substances, technically feasible. Third edition, VCH Publishers, Inc. New York, NY, The reduction of Sb 2S3 with carbon in the presence (1994). of CaO occurs through the formation of SbO 2 as [21] L. B. Pankratz, A. D. Mah, S. W. Watson, intermediate compound, which is reduced Thermodynamic Properties of Sulfides, US Bureau of subsequently by CO(g) to yield antimony metal. Mines, Bulletin 689 (1987) 334. The overall reduction reaction is: [22] P. L. Walker, M. Shelef, R. A. Anderson, Chemistry and Physics of Carbon, Marcel Dekker, New York, NY, Sb 2S3 + 3CaO + 1.5C → 2Sb + 3CaS + 1.5CO 2(g) (1968), Vol. 4, 287-301. Temperature has a large influence on the reduction [23] E. T. Turkdogan, J. V. Vinters, Carbon, 8 (1970) 39-53. rate. Complete conversion of Sb 2S3 was obtained at [24] Y. K. Rao, B. P. Jalan, Metall. Trans., 3 (1972) 2465-77. 1123 K in less than 250 s using sample mixtures with molar ratio of Sb 2S3:CaO:C equal to 1:3:3. At temperatures higher than 1123 K, the vaporization of the antimony product becomes